This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-012108, filed Jan. 20, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a color Doppler ultrasound diagnosis apparatus and, more particularly, to an improvement in the performance of a power Doppler image for displaying power from a bloodstream.
As a method of two-dimensionally displaying a bloodstream, a color Doppler method is available. In the color Doppler method, a Doppler shift (frequency shift) caused by the flow of blood is extracted, and three types of bloodstream information, i.e., an average flow rate, dispersion, and power, are generated from the Doppler shift. A two-dimensional image associated with this bloodstream information is displayed in color. Typically, this color image is superimposed on a monochrome tomographic image (B-mode image).
Note that a method of displaying the power information of a bloodstream is called a power Doppler method, and a method of displaying the flow rate and/or dispersion information of the bloodstream is called a color Doppler method; these two methods are discriminated from each other.
Power Doppler is superior in sensitivity and resolution over color Doppler, and hence tends to be preferentially used as a method of displaying a region other than the heart.
In power Doppler (as well as in color Doppler), the frame rate (time resolution) is much lower than in B-mode. In addition, power Doppler is inferior in distance resolution to B-mode.
There are several reasons why power Doppler is inferior in distance resolution to B-mode. The main reason is that a long wave train length is set.
The main reason why color Doppler and power Doppler are inferior in time resolution to B-mode is that ultrasound pulses are repeatedly transmitted/received in the same direction in color Doppler and power Doppler. Transmission/reception is repeated 8 to 20 times generally, and 16 times typically. The number of times of this operation is termed as an ensemble size. If this ensemble size is small, clutter cannot be completely removed by a wall filter. For this reason, the ensemble size cannot be set to be very small. Therefore, an improvement in time resolution cannot be expected.
Recently, a great deal of attention has been paid to a contrast echo method which performs ultrasound diagnosis by injecting an ultrasound contrast medium mainly consisting of microbubbles intravenously. Visualizing methods for this method are roughly classified into three methods, namely a harmonics B-mode method, a general power Doppler method using fundamental waves, and a harmonics power Doppler method. Of these three methods, the power Doppler method using fundamental waves exhibits the highest bloodstream sensitivity. In the contrast echo method, an intra-tissue bloodstream, i.e., perfusion, is often observed. In the power Doppler method, therefore, problems often arise in terms of clutter.
In the harmonics power Doppler method, no clutter occurs. However, this method is inferior in resolution to harmonics B-mode, and exhibits no significant difference in sensitivity. For this reason, harmonics B-mode is generally used in many cases. In the contrast echo method, since echoes are enhanced by a contrast medium injected into a blood vessel, a bloodstream can be observed in B-mode.
In the contrast echo method, perfusion can be effectively visualized by setting a high MI (Mechanical Index) value representing the power of ultrasound waves in an object. In general, reflected echoes from an intra-tissue bloodstream are small. However, strong reflected echoes can be generated by destroying microbubbles by transmitting ultrasound waves with a high MI value. On the other hand, since ultrasound waves with a high MI value destroy most microbubbles, next ultrasound transmission must be performed after fresh microbubbles are injected. For this purpose, a so-called flash echo method has been developed, which intermittently repeats ultrasound transmission with a high MI value. This flash echo method is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 8-280674. This flash echo method, however, exhibits a low time resolution.
In order to remove this demerit, a power pulse inversion method has been developed. In this method, ultrasound waves are transmitted with a low MI value so as not to destroy many bubbles. A decrease in sensitivity with a decrease in MI value is compensated for by removing clutter of fundamental waves and using both a fundamental wave and harmonics as bloodstream signals. That is, in this method, since a bloodstream is visualized by using both harmonics and fundamental wave, the sensitivity is higher than that in the conventional harmonics Doppler method. In addition, since the bloodstream drawing sensitivity in the power Doppler method is higher than that in B-mode, perfusion can be observed by continuously transmitting bubbles without destroying bubbles with a low MI value. However, the resolution is lower than that in bloodstream display in B-mode.
In the contrast echo method, to improve the distance resolution, attempts have been made to set the burst wave number in power Doppler to 1 to 2 as in B-mode. However, in this method the distance resolution is lower than that in B-mode.
The method of directly imaging a bloodstream in B-mode is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-318902. In this method, a tissue image and bloodstream image are simultaneously displayed by partly filtering some of components in a B-mode band (near DC). In this method, since a tissue image and bloodstream image are completely generated from the same pulses, it is difficult to satisfy both requirements for the resolution of a tissue image (high-frequency, broadband pulses are effective) and penetration of a bloodstream image (low-frequency, narrowband pulses are effective).
It is an object of the present invention to improve the distance resolution, sensitivity, and clutter removing effect and increase the frame rate in power Doppler.
An ultrasound diagnosis apparatus includes an ultrasound probe and a beam-former configured to scan an object to be examined with ultrasound waves through the ultrasound probe. A B-mode processor generates B-mode image data on the basis of a reception signal output from the beam-former. A power Doppler processor generates power Doppler data on the basis of the reception signal output from the beam-former. The power Doppler data is generated on the basis of the reception signal acquired under transmission conditions that the burst wave number is equal or substantially equal to that for the B-mode data, and the ensemble size is set to one of 2 to 10. The B-mode image data and power Doppler data are partly synthesized.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.